On the competition between the inversion and rotation mechanisms in the cis-trans thermal isomerization of diazene

Accurate ab initio MO calculations (6−311+G(3df,2p) basis set, CAS-SCF, CIPSI technique) have been carried out to estimate the relation between the inversion and rotation mechanisms in the cis-trans thermal isomerization of diazene. Our results, indicating the inversion transition state to be 6–7 kcal/mol lower in energy than the rotation one, correct the recently suggested equivalence of the two reaction pathways. Despite the support for the widely accepted predominance of inversion, the estimated energy difference is smaller than frequently postulated and is further diminished when considering estimates of zero-point vibration energies and thermal corrections, thus not excluding the competition of rotation in other azo compounds derived from diazene.